Visual short-term memory (VSTM) abilities undergo rapid development in infancy. Whereas 6-month-old infants detect changes only in arrays containing one item, 8-month-old infants detect changes in arrays of 2 to 3 items (Oakes et al., 2006, 2013). Moreover, 6-month-old infants evidently fail to encode any information from multi-item arrays into VSTM (Oakes et al., 2006, 2009). The mechanism of this development between 6 and 8 months remains elusive. Here we asked whether 6-month-old infants' failure to encode any information from multi-item arrays is related to difficulties with simultaneously individuating multiple items. We tested twenty-two 6-month-old infants in a one-shot change detection procedure. On each trial, a sample array of two colored items is presented briefly (500 ms), followed by a retention period (300 ms) and then a test array consisting of two colored items (1917 ms). One test item is unchanged from the sample to test array and the other item is changed to a new color. Critically, we facilitated individuation of the two items in the sample array by staggering their onset: one item appeared alone for 100 ms, then the other item appeared and both items were visible for 400 ms, and then the first item disappeared, leaving the remaining item visible alone for 100 ms. We recorded infants' eye positions using an infrared eye tracker. Infants looked significantly longer at the changed test item than at the unchanged test item, t(21) = 4.17, p <0.0005, d = 1.82. In contrast, we previously found that 6-month-old infants looked equally at the changed and unchanged test items when the sample items appeared simultaneously (Oakes et al., 2013). Thus, a staggered onset—which presumably helps infants individuate the to-be-encoded objects—leads to improved VSTM performance in 6-month-old infants, suggesting that the ability to individuate multiple items is an integral component of VSTM development.